Function template visit_each

Synopsis

Description

The visit_each mechanism
allows a visitor to be applied to every subobject in a given
object. It is used by the Signals library to discover
signals::trackable objects within a
function object, but other uses may surface if used
universally (e.g., conservative garbage collection). To fit
within the visit_each framework,
a visit_each overload must be
supplied for each object type.

The third parameter is
long for the fallback version
of visit_each and the argument
supplied to this third paramter must always be 0. The third
parameter is an artifact of the widespread lack of proper
function template ordering, and will be removed in the future.

Library authors will be expected to add additional
overloads that specialize the T argument for their classes, so
that subobjects can be visited.

Calls to visit_each are required to be unqualified, to
enable argument-dependent lookup.